The present invention concerns a technique for refining a porous coal of low rank into an excellent porous coal for solid fuel and, more in particular, it relates to a technique of efficiently drying a porous coal which is considered have only a of low economical value due to high water content, effectively preventing spontaneous combustion caused upon drying and obtaining a porous coal for solid fuel provided with increased calorific value due to dewatering and deposition of a heavy oil fraction.